In an oversampled digital to analogue (D/A) converter, such as a sigma-delta modulator, a digital input signal is to be converted to an analogue output signal. The input signal is first interpolated to produce an oversampled signal, which is modulated by a digital modulator to generate an N-bit signal. The N-bit signal has a low-frequency component resembling the input signal and a high-frequency "quantization noise", which is typically removed with a low pass filter.
For an N-bit D/A converter there must be at least one (N=1) dynamic analogue element, such as a current source, which is used to generate the analogue signal. Each additional element used gives an increase in signal to noise ratio (SNR) of 6 dB, so it is desirable to have a large N if possible.
However, a problem with this arrangement is that each analogue element will be slightly mismatched with respect to the others, and the oversampling gives rise to harmonic distortion if the elements, which are arranged in an array, are always used in the same order.
Methods of dynamic element matching are known, which seek to disrupt the order of using the elements by selecting a new starting position in the array, based on the last used element (such that the elements are used substantially equally). However, for some signal levels, the above matching method gives rise to a pattern of element usage that produces low frequency tone.
This invention seeks to provide a noise cancellation circuit which mitigates the above mentioned disadvantages.